Even though it says "initial back-end language(s)" it looks like the information is often pulled from the startup's current career pages. Mixpanel is listed as

Even though it says "initial back-end language(s)" it looks like the information is often pulled from the startup's current career pages. Mixpanel is listed as using Go as one of their initial back-end languages, even though Mixpanel was founded in June 2009, several months before the first version of Go was released.